The present invention relates generally to compositions and methods for producing prolonged pain relief and more particularly to injectable local anesthetics for that purpose.
Intractable pain, such as that suffered by cancer patients or patients with back problems, is a serious clinical problem in that it oftentimes does not respond to conventional modes of therapy. Some modes of treatment, although relieving pain, produce undesirable side effects. An example thereof is the use of epidural phenol injections to treat intractable cancer pain. This method is usually effective, but it oftentimes produces motor weakness and loss of bowel and bladder control as possible complications.
There is a group of local anesthetic compounds, comprising ethyl aminobenzoate (benzocaine) and butyl aminobenzoate (butamben), characterized by the fact that its members are poorly soluble in water. Due to this insolubility, the compounds are not absorbed with sufficient rapidity to be toxic and therefore can be applied directly to surface wounds and ulcerated surfaces. For the same reason, these compounds remain localized at the site of application for long periods of time, producing a sustained anesthetic action.
The aforementioned water-insoluble, local anesthetics are soluble in non-aqueous media such as oils or glycerol, but solutions of this type cannot be injected because of severe nerve damage that can result from such a solvent.
There is a book entitled "Neural Blockage In Clinical Anesthesia And Management Of Pain", edited by Michael J. Cousins et al, J. B. Lippincott Company, Philadelphia, 1980. Chapter 4 therein by Scott and Cousins is entitled "Clinical Pharmacology of Local Anesthetic Agents", and at page 102 it discloses the use of benzocaine as an injectable local anesthetic, either dissolved in urethane or suspended in dextran, the latter being preferable as a carrier because urethane is now known to be carcinogenic. Because of its relative insolubility in water, injected benzocaine (2% dissolved in urethane) reportedly remains at the injection site for a time which provides a nerve block that lasts many hours or even days.
Water-soluble local anesthetics are of limited pain relieveing duration, relief being no more than a matter of hours.
Polyethylene glycol has been used as a suspending agent for steroids when the latter have been injected as an anti-inflammatory agent.